Global warming is a serious issue for all humankind. In order to delay and stop the progress of the warming, every country is trying to promote energy savings by building smart grids and promote large-scale introduction of natural energies that do not discharge carbon dioxide, such as photovoltaic power generation and wind power generation.
The wind power generation has a merit of not discharging carbon dioxide by utilizing a natural energy; however, there are concerns about power quality degradation as an adverse effect on a power system because the output of power generation is dependent on wind conditions and accordingly unstable. For preventing such an adverse effect on the power system and effectively utilizing the energy, expectations are rising for wind power generation and storage systems using lead storage batteries or the like.
A storage battery system used to reduce fluctuations in the output of wind power generation makes the storage batteries charged and discharged to smooth the output of the wind power generation fluctuating significantly in a short period of time. This combines the outputs of the wind power generation and storage battery system, thereby supplying stable electric power to the power system.
The storage battery system is required to have a long life roughly equivalent to that of the wind power generator and to be installed at low cost.
The storage batteries used in the storage battery system are operated in a PSOC (Partial State of Charge) so as to be charged and discharged in accordance with the output fluctuations of the wind power generation. Accordingly, the storage batteries are not fully charged in a normal operation state, different from conventional storage batteries for emergencies (that are fully charged normally and discharged when needed) and industrial storage batteries (that are fully charged at night and discharged when the load is heavy in the daytime). Because of the special usage, an equalization charge (recovery charge) is regularly performed (usually once every one or two weeks) on the storage batteries to make the storage batteries fully charged.
Patent Literature 1 (PTL 1) discloses an example in which the frequency of applying equalization charges to storage batteries is changed in accordance with ambient temperature. Patent Literature 2 (PTL 2) discloses a method for preventing degradation of positive electrodes of storage batteries by setting the amount of the overcharge to a lower value (99% to 102%) than the conventional one (110% to 115%) upon performing an equalization charge on the storage batteries.
Patent Literature 3 (PTL 3) discloses a storage battery control system for wind power generation. The system assesses a relationship between an operation, and a lifetime and a degradation of storage batteries based on data collected by a storage battery operation and degradation data collection unit, makes a plan of how to operate the storage batteries to satisfy necessary lifetime requirements based on the obtained information, and operates the storage batteries in accordance with the plan.
Patent Literature 4 (PTL 4) discloses a storage battery device that estimates a present state of batteries by using multi-dimensional characteristic models in which a plurality of relational models between a terminal voltage and current measured, and a state of charge are prepared for each of degradation degrees.
Non-Patent Literature 1 (NPTL 1) discloses a method for preparing an SOC model (a discharge model) that represents a relationship between current, voltage, and temperature of a lead storage battery, and a state of (SOC) charge of the lead storage battery.